“Donald Trump talks tough about China and Mexico, but who has Trump ever actually taken on?” a narrator in the ad says. “He hides behind bankruptcy laws to duck paying his bills and kill American jobs. He even tried to kick an elderly woman out of her home through eminent domain. Real tough guy.”

This is the second Trump attack ad set to air in the battleground state that holds its election on March 15. The super PAC, Our Principles PAC, created in mid-January to specifically to take down Trump, announced an ad in Florida as part of a “seven figure” bundle Tuesday that knocks Trump for Trump University, calling the now-defunct for-profit real estate school a “scam.”

Some of the major donors behind Our Principles PAC, which was started by Mitt Romney's 2012 deputy campaign manger Katie Packer, hosted a conference call for fellow Republican donors Tuesday in an effort to convince them to open their checkbooks, insisting that it’s not too late to stop Trump.

The conference call was organized by a trio of high-dollar GOP donors, including Chicago Cubs co-owner Todd Ricketts, an initial backer of the effort who has given as much as $3 million, according to reports.

Both Club for Growth and Our Principles PAC argue that their efforts have paid off. The Club points to Trump’s second place finish in Iowa and Oklahoma, two states where they aired negative ads against Trump.

“Donald Trump’s loss last night in Oklahoma and his razor thin margin in Arkansas are due, in no small part, to CFG Action’s television ads that tell the truth about Trump,” said Club for Growth president David McIntosh. “We’ve used a simple and effective method of exposing the most liberal, big-government candidate in the Republican race.”

Some Republican donors are skeptical, however, of any moneyed effort to stop Trump this late in the game.

John Ying, chairman of the Asian Republican Coalition and a member of Jeb Bush’s presidential finance committee, said none of the controversial statements Trump has made have derailed his campaign. He doesn’t think a negative ad campaign will impact him either.

“It’s not clear to me that people start going after him with hammer and tongs that it will be effective,” he told NBC News Wednesday.

But time is running out. Trump has won 325 delegates compared to Sen. Ted Cruz, who is in second place and has won 237 in the first 15 contests. A majority of states will have voted by March 15 when delegate rich states of Florida, Ohio and Illinois hold their elections.

Florida is a must-win state for its senator, Marco Rubio. Rubio has won just one state out of 15 and his path to the nomination runs through Florida. Trump is currently leading Rubio in the polls there by 20 points.